1. Field of the Invention
This invention relates to spa control systems and, more particularity, to methods of measuring water flow through the heater of a spa, reporting flow status to the user, and monitoring spa water temperature in an energy-efficient manner.
2. Discussion of Related Art
For several years spa manufactures have been using two or more solid-state sensors to monitor water temperature in the spa as well as temperature somewhere near the heater. One sensor is needed to monitor temperatures at the heater according to the requirements in UL 1563, a standard for electric spas. Another sensor is usually located in the water of the spa to measure the temperature of the spa water.
In conjunction with solid-state sensors, a flow-monitoring device has also commonly been used. The spa industry has long used pressure switches in the plumbing as an indication that the circulation pump is running and water is present. This usage of pressure switches has the drawback that certain types of blockage can stop the flow of water but still indicate pressure in the plumbing from the pump. A better plan has been the usage of flow switches. Many spas being built today employ a flow switch to determine if it is appropriate to activate the heater. Flow switches are somewhat expensive, however, and often unreliable.
U.S. Pat. No. 5,361,215, Tompkins, et al, teaches the use of two temperature sensors to determine water flow though the heater. One sensor is upstream from the heater while the second sensor is downstream from the heater. A significant difference in temperature between the two sensors is an indication of a flow problem. In all cases, one of the sensors is in the spa water. The other sensor is near the heater. U.S. Pat. No. 6,282,370, Cline, et al, teaches the use of two sensors at separated locations on or within the heater to determine adaquate water flow through the heater and also to measure the temperature of the water in the spa. Again, the difference in temperature between the two sensors is used to evaluate the presence of water flow of through the heater.
The Cline approach has several disadvantages. The first problem is that the difference in temperature between the two sensors is very small, even with significant blockage in the plumbing. The Cline approach can be accurate only when the water flow is above some minimum level. This approach cannot, therefore, be used with low-flow heaters, which are popular in the spa industry. Another problem is that the spa water temperature is not known when the pump is off. The only way to learn the water temperature is to turn on the pump for a short period several times a day in order to measure the water temperature as it passes through the heater and to see if heat is needed. Clearly, this approach is not energy friendly.